Publications

Artificial multilayers for neutron optical applications can be produced reliably in large quantities using pulsed DC-magnetron sputtering techniques. Co-sputtering and reactive sputtering are effective means for minimizing interdiffusion and build-up of roughness at interfaces. Moreover, for polarizing mirrors, the scattering contrast between the magnetic and non-magnetic layers can be adjusted in a simple way. The achieved perfection of the multilayers is now so excellent that the surface morphology of the substrates (glass, Si) becomes a major concern. Supermirrors have been produced with up to four times the critical angle of bulk Ni and we discuss their application for transporting and focussing neutron beams. Using the effect of magnetostriction, it is possible to build spin selective devices with high transmission composed of remanent polarising mirrors.

We built a compact (25 cm long) focusing device for neutron diffraction studies of very small samples (mm³ or less). Neutrons are reflected on Ni/Ti supermirror surfaces both in the horizontal and in the vertical plane. The focusing angle in the horizontal plane is variable to choose an optimal ratio between intensity and angular resolution. A gain in intensity of four times was obtained. The new stage of the focusing system should provide a gain up to 7 times.

The performance of instruments using neutron beams can be significantly improved by optimizing all the neutron optical components that are responsible for the transport of the “good” neutrons from the source to the sample and to the detector. The principal interaction of neutrons with matter is of nuclear and magnetic origin. Therefore, most devices use these interactions for defining the phase space of the neutrons (energy, divergence), either by diffraction from bent crystal monochromators or by reflection from artificial multilayers at grazing incidence. In order to define the spin degrees of freedom, polarizing mirrors and monochromators, or absorbers (3He) are used. In this contribution we discuss neutron optical devices based on crystal monochromators and artificial multilayers.

In order to study radiation stability of the neutron supermirror coatings, the effect of large irradiation doses on polarizing (FeCoV/TiZr) and non-polarizing (NiMo/Ti and Ni/Ti) supermirrors has been studied. The samples were irradiated at the WWR-M reactor (Gatchina) inside a vertical channel behind the Be reflector, where the flux of thermal neutrons and neutrons with energies exceeding 1 MeV was, respectively, (0.6–1)×10¹³ and 2×10¹¹ cm⁻² s⁻¹. During irradiation the temperature of the samples did not exceed 70°C. The data obtained at the reflectometer RPN-2M (PNPI) for FeCoV/TiZr and NiMo/Ti supermirrors (produced in PNPI) and for Ni/Ti supermirrors (produced in PSI), irradiated with different doses, are presented. The polarizing efficiency and the reflectivity of the supermirrors sputtered onto float-glass substrates have not revealed significant changes for the fluences up to 10¹⁹ cm⁻².

Modern materials can often only be grown in small quantities. Therefore, neutron-scattering experiments are difficult to perform due to the low signal. In order to increase the flux at the sample position, we have developed the concept of a small focusing guide tube with parabolically shaped walls that are coated with supermirror m=3. The major advantage of parabolic focusing is that the flux maximum occurs not at the exit of the tube. It occurs at the focal point that can be several centimeters away from the exit of the tube. We show that an intensity gain of 6 can easily be obtained. Simulations using the software package McStas demonstrate that gain factors up to more than 50 can be realised on a spot size of approximately 1.2 mm diameter. For PGAA we expect flux gains of up to three orders of magnitude if multiplexing is used. We show that elliptic ballistic guides lead to flux gains of more than 6.

Sophisticated neutron guide systems take advantage of supermirrors being used to increase the neutron flux. However, the finite reflectivity of supermirrors becomes a major loss mechanism when many reflections occur, e.g. in long neutron guides and for long wavelengths. In order to reduce the number of reflections, ballistic neutron guides have been proposed. Usually linear tapered sections are used to enlarge the cross-section and finally, focus the beam to the sample. The disadvantages of linear tapering are (i) an inhomogeneous phase space at the sample position and (ii) a decreasing flux with increasing distance from the exit of the guide. We investigate the properties of parabolic and elliptic tapering for ballistic neutron guides, using the Monte Carlo program McStas with a new guide component dedicated for such geometries. We show that the maximum flux can indeed be shifted away from the exit of the guide. In addition we explore the possibilities of parabolic and elliptic geometries to create point like sources for dedicated experimental demands.

The aim of the investigation was to test the focusing properties of a new type of focusing neutron guide (trumpet) with parabolically shaped walls. The guide has a length of 431 mm with an entrance area of 16×16 mm² and an output area of 4×4 mm². The interior surfaces were coated with a supermirror-surface m=3 and due to their parabolic shape it was expected that an incident parallel beam can be focused in the focal point of the parabolas. To prove this statement the neutron intensity distribution at different distances behind the guide was recorded by means of a standard, high-resolution radiography detector. The experiments were performed at the V12b instrument at HMI with different levels of beam monochromatization demonstrating maximum intensity gains of about 25. The consideration for using the focusing guide for the purposes of cold neutron radiography will be presented.

We have developed a multichannel parabolic guide for thermal neutron beam focusing. The guide was designed with multichannel structure, seven channels in the present case, to focus a neutron beam with wide cross section effectively. The multichannel structure is made with parabolically bent NiC/Ti supermirrors with critical reflection angles m = 3.0 and 3.6 times as much as that of nickel and thickness of 0.55 mm. The guide consists of two focusing devices with each length of 900 mm, which are separated by a gap of 150 mm for neutron beta decay experiments. The focal length of the guide is 995 mm away from the exit of the guide. The cross section of the guide is 90 × 20mm² at the entrance and 52.3 × 12.1mm² at the exit. The guide was installed at a thermal neutron beam line for neutron-induced prompt gamma-ray analysis (PGA) of JRR-3M in JAEA with straight supermirror guides with m = 3.0. Focusing properties of the parabolic guide were measured at the focal point with a CCD imaging system and we obtained a gain of 9.6 and 4.1 in peak flux compared with those in cases of using the vacuum tube and the straight guide, respectively. The parabolic guide has been successfully used to enhance the efficacy of PGA and have also applied to neutron beta decay experiments.

With the progress made in establishing new coating and grinding techniques and the availability of new simulation tools significant progress has been made in increasing the performance of optical components being used for neutron instrumentation. These include passive components like honeycomb lenses, focusing collimators, devices using supermirror coatings as well as active phase space transformers. In this contribution I shall discuss various possibilities of how the phase space of neutron beams can be adapted to match the needs of neutron beam lines and how to transport the neutrons efficiently from the moderator to the sample and detector. It is shown that elliptic guides can lead to significant flux gains while improving the resolution of spectrometers. The power of the new focusing techniques will be demonstrated for triple axis, time of flight and spin-echo spectroscopy as well as for imaging with neutrons.

The development, installation and commissioning of a compact neutron focussing device has recently been completed on the SXD single crystal diffractometer at ISIS. Such devices provide a relatively inexpensive method of significantly increasing the long wavelength neutron flux available for small samples, such as those often used on SXD, without significantly disturbing the existing beamline infrastructure. This paper will discuss the measured performance of the existing device. In addition, the development of further focussing devices suitable for the ISIS instrument suite will be discussed.

The high-resolution powder diffractometer (HRPD) has been upgraded with the installation of a 100 m supermirror guide. The guide is a unique ballistic design that is elliptic in both horizontal and vertical cross-section and utilizes a graded supermirror coating of between m = 1.5 in the center to m = 3 at the beam entry and exit points. Flux gains are considerable when compared to the performance of the original m = 1 guide. The shallower radius of curvature (36 km) of the new guide also permits transmission of short wavelength neutrons previously cut off. The flux gains have been achieved whilst retaining the high resolution at backscattering, Δd/d ∼ 5×10⁻⁴. Results following initial commissioning studies are described highlighting the new capabilities of the instrument and new scientific opportunities resulting from the upgrade are discussed.

We have designed and built a parabolically shaped reflector with a Ni/Ti-multilayer coating, graded along the length of the device. The bilayer thickness follows the varying angle of incidence to match the Bragg condition for a beam parallel to the axis of the parabola. As measurements show, this concept leads to a higher reflectivity for monochromatic neutrons compared to a conventional supermirror coating. With a reflectivity R > 96% we measured a gain of 5 when focusing a 8.6 mm wide beam of 0.1⁰ divergence down to 1.6 mm. The wavelength band was 4.7 ű5% and the focal point was 250 mm behind the device.

In the last decade the performance of neutron guides for the transport of neutrons has been significantly increased. The most recent developments have shown that elliptic guide systems can be used to focus neutron beams while simultaneously reducing the number of neutron reflections, hence, leading to considerable gains in neutron flux. We have carried out Monte-Carlo simulations for a new triple-axis spectrometer that will be built at the end position of the conventional cold guide NL-1 in the neutron guide hall of the research reactor FRM-II in Munich, Germany. Our results demonstrate that an elliptic guide section at the end of a conventional guide can be used to at least maintain the total neutron flux onto the sample, while significantly improving the energy resolution of the spectrometer. The simulation further allows detailed insight how the defining parameters of an elliptic guide have to be chosen to obtain optimum results. Finally, we show that the elliptic guide limits losses in the neutron flux that generally arise at the gaps, where the monochromator system of the upstream instrument is situated.

Following the border of antiferromagnetism (AF) to zero temperature is a promising route to unconventional metallic and superconducting phases. Many interesting examples of antiferromagnetic quantum phase transitions can only be reached by pressure tuning. The range of quantitative experimental probes, which can be realised in a high-pressure environment is limited. However, advances have recently been made in neutron scattering, where elliptically shaped neutron guides now increase the beam intensity directed to mm size sample for high pressure studies. This has been demonstrated on the simple antiferromagnet NiS₂NiS₂. Neutron scattering also allows highly accurate measurements of the lattice constant via the Larmor diffraction technique, which proved extremely useful in studying the high-pressure phase diagram of the itinerant helimagnet MnSi. We now combined Larmor diffraction with conventional diffraction measurements to investigate the pressure–temperature phase diagram of URu₂Si₂URu₂Si₂ up to 20 kbar. URu₂Si₂URu₂Si₂ offers a further spectacular example for the presence of unconventional phases in the vicinity of antiferromagnetism. In this compound, antiferromagnetism is replaced below approximately 5 kbar by the mysterious “hidden order” (HO) and unconventional superconductivity. Our measurements allow the observation of magnetic order and changes in the aa- and cc-axis lattice constants across the phase transitions in the same experiment. The results contain clear indications of a first-order transition and strong differences between the AF phase and the HO phase in the coupling to the lattice.

Recently, we have developed an optimized process for polishing Al substrates leading to an extraordinary low surface roughness comparable to float glass of high quality. Indeed, supermirror coatings with R = 91% at the critical angle of reflection of m = 2 were produced demonstrating the excellent quality of the Al surface. Supermirror coated Al substrates open new options for advanced neutron optical devices. As one example neutron guides can start very close from the moderator. We investigated this option by Monte-Carlo simulations for i) a conventional, curved guide and ii) an elliptic guide. In case of i) an increase of flux at long wavelengths is obtained because of complete illumination of the phase space accepted by the guide. The elliptic guide starting close to the moderator (ii) has significantly enhanced neutron transport properties since only useful neutrons are extracted and transported to the sample. As a result such a guide concept is superior in terms of flux when compared with a conventional guide system and in terms of signal to background and well defined beam focusing compared with an elliptic guide starting at a larger distance from the moderator. In particular, experiments on novel materials, which are often only available in small quantities or samples under extreme conditions will profit from neutron beams of such high quality.

With the invention of elliptic guides, the neutron flux at instruments can be increased significantly even without sacrificing resolution. In addition, the phase space homogeneity of the delivered neutrons is improved. Using superpolished metal substrates that are coated with supermirror, it is now possible to install neutron guides close to the moderator thus decreasing the illumination losses of the guide and reducing the background because the entrance window of the elliptic guide can be decreased significantly. We have performed Monte Carlo simulations using the program package MCNP5 to calculate the shielding requirements for an elliptic guide geometry assuming that the initial guide section elements are composed of Al substrates. We show that shielding made from heavy concrete shields the neutron and γ-radiation effectively to levels below 1 μSv/h. It is shown that the elliptic geometry allows to match the phase space of the transported neutrons easily to the needs of the instruments to be installed. In particular it is possible to maintain a compact phase space during the transport of the neutrons because the reflection losses are strongly reduced.

In various fields of neutron scattering there is a tendency to use smaller and smaller samples. There are various reasons for this, e.g. the limited size in high pressure cells, the restrictions given by growth methods of thin films, or the impossibility to grow larger single crystals. With conventional guides this leads to the situation that a white beam with some 50 cm² cross-section and a broad divergence is to illuminate a sample of some mm² area. Thus more than 99% of the neutrons leaving the guide are not needed and cause background and radiation problems.

It is suggested to change the order of the optical elements and the design of the guide section to filter neutrons not intended to hit the sample as early as possible. As an example a set-up for specular reflectivity on small samples is presented. A double monochromator some meters behind the source cuts away all neutrons of the wrong wavelength even before they enter the guide. The guide itself is one branch of an ellipse. It maps the divergent beam from the monochromator to a convergent beam at the sample position. An entry aperture at the first focal point, a bit larger than the sample, guarantees that just enough neutrons enter the guide to bath the sample. There is no direct line of sight to the source and the guide ends far away from the sample position, so that there are only few spacial restrictions.

Detailed McStas calculations and a design study for a down-scaled test device, both for reflectometry and diffraction, are presented.

We present Monte-Carlo simulations for the focusing design of a novel cold-neutron triple-axis spectrometer to be installed at the end position of the cold guide NL-1 of the research reactor FRM-II in Munich, Germany. Our simulations are of general relevance for the design of triple-axis spectrometers at end positions of neutron guides. Using the McStas program code we performed ray trajectories to compare parabolic and elliptic focusing concepts. In addition the design of the monochromator was optimized concerning crystal size and mosaic spread. The parabolic focusing concept is superior to the elliptic alternative in view of the neutron intensity distribution as a function of energy and divergence. In particular, the elliptical configuration leads to an inhomogeneous divergence distribution.

In many areas of soft and hard matter research science, the amount of material to investigate is rather limited. Partly because the fabrication of larger samples is too expensive or not feasible, yet, partly because the interesting features depend on the size. The aim of this work is to develop a neutron reflectometer optimized for small samples and specular measurements. We present a new concept which is based on state-of-the-art neutron optical elements and which allows studies of samples 100 times smaller than with previous instrumentation. The concept is to use an elliptically focusing guide in the sample plane, and a converging beam geometry in the scattering plane. The latter allows for the simultaneous measurement of a wide angle-of-incidence range simultaneously. Here we report on the prototype setup and first measurements, where we reached a reduction of counting time by one order of magnitude. If a complete instrument is built based on the presented principle, another order of magnitude can be expected.

Neutron strain diffractometers usually use slits to define a gauge volume within engineering samples. In this study a multi-channel parabolic neutron guide was developed to be used instead of the primary slit to minimise the loss of intensity and vertical definition of the gauge volume when using slits placed far away from the measurement position in bulky components. The major advantage of a focusing guide is that the maximum flux is not at the exit of the guide as for a slit system but at the focal point relatively far away from the exit of the guide. Monte Carlo simulations were used to optimise the multi-channel parabolic guide with respect to the instrument characteristics of the diffractometer STRESS-SPEC at the FRM II neutron source. Also the simulations are in excellent agreement with experimental measurements using the optimised multi-channel parabolic guide at the neutron diffractometer. In addition the performance of the guide was compared to the standard slit setup at STRESS-SPEC using a single bead weld sample used in earlier round robin tests for residual strain measurements.

As novel materials of excellent homogeneity can often only be grown in small quantities it is important to optimize the transport of neutrons from the moderator to the sample while keeping the background low. Using elliptically or parabolically tapered guides the losses can be strongly reduced such that 50% – 90% of the useful neutrons arrive at the sample. If not properly designed, however, the divergence at the sample becomes inhomogeneous. In contrast, pairs of nested Kirkpatrick-Baez mirrors in Montel geometry yield well focused beams with a compact phase space. The mirrors extract only the useful neutrons from the moderator and effectively interrupt the line of sight leading to a very low background. As the focal distances are typically several meters, the extraction of the neutrons and the installation of bulky sample environment is facilitated.

SPICA, a new special environment powder neutron diffractometer was built at BL09 in the Material and Life science Facility (MLF) of the Japan Proton Accelerator Research Complex (J-PARC). This is the first instrument dedicated solely to the study of next-generation batteries in J-PARC and is optimized for in situ measurements to clarify structural changes of materials in batteries. The basic design and instrumentation of SPICA have been completed. The highest Δd/d resolution achieved at the commissioning stage was 0.09% at the back scattering bank of SPICA. The reliability of the diffraction data has achieved a sufficiently high level for the structural analysis of materials using the Rietveld method. The air scattering banks with the blades made of B4C for in situ measurements also function very well.

We report the development of a versatile module that permits fast and reliable use of focussing neutron guides under varying scattering angles. A simple procedure for setting up the module and neutron guides is illustrated by typical intensity patterns to highlight operational aspects as well as typical parasitic artefacts. Combining a high-precision alignment table with separate housings for the neutron guides on kinematic mounts, the change-over between neutron guides with different focussing characteristics requires no readjustments of the experimental setup. Exploiting substantial gain factors, we demonstrate the performance of this versatile neutron scattering module in a study of the effects of uniaxial stress on the domain populations in the transverse spin density wave phase of single crystal Cr.

In view of the trend towards smaller samples and experiments under extreme conditions it is important to deliver small and homogeneous neutron beams to the sample area. For this purpose, elliptic and/or Montel mirrors are ideally suited as the phase space of the neutrons can be defined far away from the sample. Therefore, only the useful neutrons will arrive at the sample position leading to a very low background. We demonstrate the ease of designing neutron transport systems using simple numeric tools, which are verified using Monte-Carlo simulations that allow taking into account effects of gravity and finite beam size. It is shown that a significant part of the brilliance can be transferred from the moderator to the sample. Our results may have a serious impact on the design of instruments at spallation sources such as the European Spallation Source (ESS) in Lund, Sweden.

Neutron Spectroscopy employing extreme-conditions sample environments is nowadays a crucial tool for the understanding of fundamental scientific questions as well as for the investigation of materials and chemical-physical properties. For all these kinds of studies, an increased neutron flux over a small sample area is needed. The prototype of a focusing neutron guide component, developed and produced completely at the neutron source FRM II in Garching (Germany), has been installed at the time-of-flight (TOF) disc-chopper neutron spectrometer TOFTOF and came into routine-operation. The design is based on the compressed Archimedes‘ mirror concept for finite-size divergent sources. It represents a unique device combining the supermirror technology with Adaptive Optics, suitable for broad-bandwidth thermal-cold TOF neutron spectroscopy (here optimized for 1.4–10 Å). It is able to squeeze the beam cross section down to a square centimeter, with a more than doubled signal-to-background ratio, increased efficiency at high scattering angles, and improved symmetry of the elastic resolution function. We present a comparison between the simulated and measured beam cross sections, as well as the performance of the instrument within real experiments. This work intends to show the unprecedented opportunities achievable at already existing instruments, along with useful guidelines for the design and construction of next-generation neutron spectrometers.

We demonstrate the performance of a compact neutron guide module which boosts the intensity in inelastic neutron scattering experiments by approximately a factor of 40. The module consists of two housings containing truly curved elliptic focussing guide elements, positioned before and after the sample. The advantage of the module lies in the ease with which it may be reproducibly mounted on a spectrometer within a few hours, on the same timescale as conventional sample environments. It is particularly well suited for samples with a volume of a few mm3, thus enabling the investigation of materials which to date would have been considered prohibitively small or samples exposed to extreme environments, where there are space constraints. We benchmark the excellent performance of the module by measurements of the structural and magnetic excitations in single crystals of model systems. In particular, we report the phonon dispersion in the simple element lead. We also determine the magnon dispersion in the spinel ZnCr2Se4 (V = 12.5 mm3), where strong magnetic diffuse scattering at low temperatures evolves into distinct helical order.

The cold-neutron triple-axis spectrometer PANDA at the neutron source FRM II has been serving an international user community studying condensed matter physics problems. We report on a new setup, improving the signal-to-noise ratio for small samples and pressure cell setups. Analytical and numerical Monte Carlo methods are used for the optimization of elliptic and parabolic focusing guides. They are placed between the monochromator and sample positions, and the flux at the sample is compared to the one achieved by standard monochromator focusing techniques. A 25 times smaller spot size is achieved, associated with a factor of 2 increased intensity, within the same divergence limits, . This optional neutron focusing guide shall establish a top-class spectrometer for studying novel exotic properties of matter in combination with more stringent sample environment conditions such as extreme pressures associated with small sample sizes.

Supermirrors with large critical angles of reflection, i.e. large index m are an essential ingredient to transport, focus and polarise neutrons over a wide range of energy. Here we summarise the recent developments of supermirror with very large critical angles of reflection and high reflectivity that were conducted at SwissNeutronics as well as their implementation in devices. Approaching critical angles m = 8 times the critical angle of natural nickel makes new applications possible and extends the use of reflection optics towards the regime of hot and epithermal neutrons. Based on comparisons of simulations with experiment we demonstrate future possibilities of applications of large-m supermirrors towards devices for neutrons with short wavelength.

We have developed a variable focusing system for neutrons that allows varying the vertical and horizontal beam size at the sample position independently. It is based on a focusing parabolic neutron guide coated with supermirror m = 6 times the critical angle of reflection of Ni. The divergence of the incident beam is adjusted by solid-state collimators with horizontally and vertically arranged absorbing blades. The performance of the prototype set-up has been benchmarked at the beamline BOA at SINQ confirming the expected behavior. In particular we show that the beam size at the sample position can be varied between 0.6 and 2 mm without exchanging the focusing guide or by introducing a beam-defining aperture between the exit of the guide and the sample, i.e. the beam size can be adapted more than half a meter away from the sample.

The quantification of the dose rate and the composition of the dose in the vicinity of supermirror coated guides is essential for designing shielding for beamlines transporting neutrons with a high flux. We present results on a calculation of radiative neutron absorption in Ni/Ti and NiMo/Ti supermirror coatings which leads to the emission of high energy gamma rays. A simple parameterization of the absorption probability in the coating materials per incident neutron is given as a function of momentum transfer at reflection.

The interface roughness of supermirrors with m = 4 times the critical angle of nickel has been investigated by the reflection of x-rays from the back side of the mirrors. Reflectivity measurements from the front side do not contain useful information about the morphology of the layers because most photons are totally reflected from the top layer, which has typically a thickness of approximately 80 nm. Therefore, it is not straightforward to obtain information about the interface roughness of the layers underneath, which are decisive for the reflectivity of the supermirrors. In contrast, specular and off-specular measurements from the back side provide quantitative information on the buildup of roughness at the interfaces as well as the lateral and vertical correlation lengths of the roughness. As the intensity of laboratory x-ray sources is much higher than the intensity of neutron beams, it is possible to probe up to 8 harmonics of the supermirror sequence corresponding to m = 32. We demonstrate that the sheets caused by resonant diffuse scattering off supermirrors with a high reflectivity have a lower intensity and larger lateral correlation lengths than mirrors with a low reflectivity. We show that the reflection of x-rays from the back side of supermirrors is an alternative method for their characterization.

The invention of self-shielding copper substrate neutron guides is described, along with the rationale behind the development, and the realisation of commercial supply. The relative advantages with respect to existing technologies are quantified. These include ease of manufacture, long lifetime, increased thermal conductivity, and enhanced fast neutron attenuation in the keV-MeV energy range. Whilst the activation of copper is initially higher than for other material options, for the full energy spectrum, many of the isotopes are short-lived, so that for realistic maintenance access times the radiation dose to workers is expected to be lower than steel and in the lowest zoning category for radiation safety outside the spallation target monolith. There is no impact on neutron reflectivity performance relative to established alternatives, and the manufacturing cost is similar to other polished metal substrates.

Neutron scattering is a well-established tool for the investigation of the static and dynamic properties of condensed matter systems over a wide range of spatial and temporal scales. Many studies of high interest, however, can only be performed on small samples and typically require elaborate environments for variation of parameters such as temperature, magnetic field and pressure. To improve the achievable signal-to-background ratio, focusing devices based on elliptic or parabolic neutron guides or Montel mirrors have been implemented. Here we report an experimental demonstration of a nested mirror optics (NMO), which overcomes some of the disadvantages of such devices. While even simpler than the original Wolter design, our compact assembly of elliptic mirrors images neutrons from a source to a target, reducing geometric aberrations, gravitational effects and waviness-induced blurring. Experiments performed at MIRA at FRM-II demonstrate the expected focusing properties and a beam transport efficiency of 72% for our first prototype. NMO seem particularly well-suited to (i) extraction of neutrons from compact high-brilliance neutron moderators, (ii) general neutron transport, and (iii) focusing and polarizing neutrons. The phase space of the neutrons hitting a sample can be tailored on-line to the needed experimental resolution, resulting in small scattering backgrounds. As additional benefits, NMO situated far away from both the moderator and the sample are less susceptible to radiation damage and can easily be replaced. NMO enable a modular and physically transparent realization of beam lines for neutron physics similar to setups used in visible light optics.

Sputtering techniques allow, by means of alloying and reactive processes, to produce artificial multilayers on a nano-meter scale with non-stochiometric compositions. Hence it is possible to adjust the index of refraction of individual layers. This property is particularly important for the fabrication of polarizing mirrors. In addition, composite layers are usually smoother and less prone to interdiffusion, i.e., more-stable with temperature. For example, Fe50 Co48 V2 /TiNx supermirrors survive temperatures as high as T ≃ 230°C. Moreover, by using anisotropic sputtering conditions, remanent mirrors can be produced with the magnetization direction pointing along any desired direction, thus allowing the fabrication of spin selectors for applications in zero field environments.

Recent developments in sputtering techniques allow the fabrication of multilayers with a high degree of perfection over large areas. We show, that using reactive sputtering, it is possible to adjust the index of refraction for neutrons, n_i, of the individual layers. This property is particularly important for polarizing mirrors, where n_nm for the non-magnetic layers can be matched to n_m of the magnetic layers such that neutrons for one spin-eigenstate are not reflected by the coating, whereas the reflectivity is high for the other spin-eigenstate. In addition, by using anisotropic sputtering conditions it is possible to orient the easy axis of magnetization within the plane of the mirrors in any particular direction resulting in a simultaneous appearance of a pronounced remanence and coercivity. Remanent polarizers can be used as broad band spin selectors at continuous and in particular at pulsed neutron sources thus eliminating the need of spin flippers, whose performance depends on the wavelength of the neutrons and is often strongly influenced by stray magnetic fields from the sample environment. The possibility to operate remanent supermirrors in arbitrary small fields leads to attractive applications of polarizing devices in low field environments such as they occur in neutron-spin-echo or in spin selective neutron guides. We present applications, where several tasks like polarizing, focusing and spin selection are performed in one single  device thus reducing the  problem of  phase  space matching between different neutron optical components.

The combination of zero-field spin-echo and triple-axis spectroscopy at a high-flux spectrometer under construction at the FRM-2 will allow the determination of the energies and lifetimes of dispersive excitations, including both phonons and magnons, over the entire Brillouin zone. We discuss the technical design of the instrument and give examples of the envisioned scientific applications.

Recent developments in sputtering techniques allow the production of polarising supermirrors with a significant in-plane magnetic anisotropy that leads to a pronounced remanent and coercive field along the easy axis of magnetisation. The magnetic layers can be magnetised by a field pulse of B_p≃20 mT either parallel or anti-parallel to a guide field B_g ≃ 1 mT. We show that polarising benders using remanent supermirrors are a versatile means to perform polarisation analysis with a three-axis spectrometer. The major advantages of remanent benders are: (i) no need for spin flippers, (ii) no cross-talk between neutron polarisation and magnetic fields at the sample position, and (iii) ideal for white beam applications.

The diffuse neutron scattering (DNS) instrument is a time-of-flight spectrometer that similar to the D7 in Grenoble has been equipped with neutron polarizers and analyzers covering a wide angular range. A particular feature of this instrument is its compact size and consequently, a focusing layout of the polarizers. The polarizing devices are benders that are made of a stack of magnetic supermirrors. Details of the instrumental design and its performance will be discussed.

For a polarized neutron beam in a neutron guide with a polarization parallel to the side walls of the guide, depolarization of the beam can occur at the top and bottom walls due to the in-plane magnetization of the Ni/Ti supermirrors. We alloyed the Ni-sublayers with Mo in order to suppress the magnetization of the layers. Magnetization measurements of the NiMo/Ti supermirrors indicate a Mo-dependency on the in-plane magnetization. For increasing Mo concentration the magnetization is decreased. Polarized neutron reflectivity measurements show no depolarizing for in-plane magnetization parallel to the neutron spin as normally is the case in the side walls of a neutron guide. A polarized neutron beam with polarization perpendicular to the magnetization of the layers is partially depolarized.

A neutron spin polarisation device with a length of only 160 mm and a cross-section of 22×34 mm² is presented. Its design is specially suited to analyse large beams typically used for radiography with polarised cold neutrons. Characterisations show a polarisation efficiency of about 78% and a transmission of 44% for neutrons with a wavelength λ of 5 Å. The compact size makes the device versatile and easily implementable for many purposes in neutron physics.

Neutron scattering with polarization analysis is an indispensable tool for the investigation of novel materials exhibiting electronic, magnetic, and orbital degrees of freedom. In addition, polarized neutrons are necessary for neutron spin precession techniques that path the way to obtain extremely high resolution in space and time. Last but not least, polarized neutrons are being used for fundamental studies as well as very recently for neutron imaging. Many years ago, neutron beam lines were simply adapted for polarized beam applications by adding polarizing elements leading usually to unacceptable losses in neutron intensity. Recently, an increasing number of beam lines are designed such that an optimum use of polarized neutrons is facilitated. In addition, marked progress has been obtained in the technology of ³He polarizers and the reflectivity of large-m   supermirrors. Therefore, if properly designed, only factors of approximately 2–3 in neutron intensity are lost. It is shown that S-benders provide neutron beams with an almost wavelength independent polarization. Using twin cavities, polarized beams with a homogeneous phase space and P > 0.99 can be produced without significantly sacrificing intensity. It is argued that elliptic guides, which are coated with large m polarizing supermirrors, provide the highest flux.

We present data on depolarizing effects in polarizing mirrors. At typical magnetizing field strengths used in polarizing devices, depolarizations rise up to the percent level in the specular region and are shown to be successfully suppressed to 10⁻⁴ when increasing the magnetizing field. We show evidence linking a part of this depolarization to lateral correlation of the magnetization fluctuations in the ferromagnetic layers. Effects of the supermirror factor (m), wavelength and incidence angle are studied. The findings are applied to a crossed supermirror geometry and we report a neutron beam polarization of 99.97(1)% for a beam of wavelength l = 5.3 Å, Dl/l = 0.1 (FWHM).

Much progress in neutron spectroscopy has been achieved with the invention of direct geometry time-of-flight (TOF) spectrometers, which allow an efficient determination of the scattering function S(Q,E) over large ranges in momentum Q and energy E. For separating (i) magnetic from nuclear and (ii) incoherent from coherent scattering, polarization analysis of the scattered neutrons is often required. Typically, the polarization is analyzed by means of He spin filters or by curved magnetized mirrors, which are arranged radially around the sample reflecting the spin-up neutrons while the spin-down neutrons are absorbed. In the following we describe the design of a wide-angle polarization analyzer that uses polarizing mirrors with an equiangular profile in transmission, i.e. the spin-down neutrons are transmitted to the detector while the spin-up neutrons are reflected and finally absorbed. Because of the almost constant angle of reflection the bandwidth is large, i.e. it is given by the ratio Q_P = m_max / m_min, where m_max and m_min @ 0.68 are the maximum and minimum useful -values of the polarizing supermirror. For small samples and m_max = 5, Q_P = 7.4 is achieved. The proposed design eliminates the significant losses of neutrons due to absorption by the front faces of the polarizing blades of wide-angle analyzers in reflection. Moreover, in transmission, the phase space of the neutrons is barely affected by the mirrors and the polarization is rather constant.

We report on a wide angle neutron spin analyzer for neutron energies up to 40 meV (l = 1.4 Å) that was recently realized in collaboration with Tohoku University and KEK, Japan (Ohoyama et al., 2014; Yokoo et al., 2014). Curved polarizing Fe/Si supermirrors with m = 5.5/1.5 (Schanzer et al., 2016) are arranged radially around the sample covering an angular range of 40° if all channels are equipped with supermirrors. The mirrors are saturated by a vertical permanent magnetic field B @ 40 mT generated by NdFeB magnets. It was optimized using finite element analysis. The polarization and transmission of the spin filter have been determined at the beamline BOA at the Swiss spallation neutron source (SINQ) to be P = (0.975±0.01) and T_up = T = (0.55±0.017) for spin-up neutrons at l = 3 Å (E = 9 meV), respectively. The transmission is excellent considering that approximately one third of the neutrons hit the front faces of the polarizing blades. The experimental results are in good agreement with Monte Carlo simulations using the software package McStas.

We report on a design study for a supermirror based neutron polarizer for a cold neutron triple-axis spectrometer to be installed at the MLZ in Garching. The spectrometer will be located at a midway position of a neutron guide, with other non-polarized instruments following downstream, such that the neutron polarizer has to be installed in the monochromatic beam downstream but close to the monochromator inside the monochromator shielding. The polarizer operates in a wavelength band 2 Å – 6 Å. The design is optimized by ray tracing simulations with the McStas software. We compare the performance of the conventional reflection and transmission type C-benders, V-bender (kink-polarizer), S-bender, and V-cavity. The latter two will only be briefly discussed here. We also propose a new design for a transmission C-bender with additional polarizing collimators.

MIEZE (Modulation of IntEnsity with Zero Effort) spectroscopy is a high-resolution spin echo technique optimized for the study of magnetic samples and samples under depolarizing conditions. It requires a polarization analyzer in between spin flippers and the sample position. For this, the device needs to be compact and insensitive to stray fields from large magnetic fields at the sample position. For MIEZE, in small angle scattering geometry, it is further essential that the analyzer does not affect the beam profile, divergence, or trajectory. Here, we compare different polarization analyzers for MIEZE and show the performance of the final design, a transmission bender, which we compare to McStas simulations. Commissioning experiments have uncovered spurious scattering in the scattering profile of the bender, which most likely originates from double Bragg scattering in bent silicon.

We report the growth of large single-crystals of Cu₂MnAl, a ferromagnetic Heusler compound suitable for polarizing neutron monochromators, by means of optical floating zone under ultra-high vacuum compatible conditions. Unlike Bridgman or Czochralsky grown Cu₂MnAl, our floating zone grown single-crystals show highly reproducible magnetic properties and an excellent crystal quality with a narrow and homogeneous mosaic spread as examined by neutron diffraction. An investigation of the polarizing properties in neutron scattering suggests a high polarization efficiency, limited by the relatively small sample dimensions studied. Our study identifies optical floating zone under ultra-high vacuum compatible conditions as a highly reproducible method to grow high-quality single-crystals of Cu₂MnAl.

Highly oriented pyrolytic graphite (HOPG) monochromators with low mosaic (grades PGCX04, PGCX05, ZYA, AGraphY, etc.) are only available up to a maximum size of 50×50 mm². In particular for one-dimensional focusing, several monochromators have to be glued or screwed on a substrate to obtain a length of typically 150 mm. The use of substrate holders such as silicon leads to additional background. By means of DC magnetron sputtering, SwissNeutronics succeeded producing assemblies of PGCX monochromators thus increasing the reflecting area of HOPG monochromators significantly while the background remainsat a low level. Therefore, the combination of the manufacturing process of Panasonic for HOPG with the sputtering technique of SwissNeutronics allows the provision of large area monochromators for customers.

We present a thorough investigation of the diffuse scattering of neutrons from Highly Oriented Pyrolytic Graphite (HOPG) that appears in the tails of the Bragg scattering profile. Using a low background neutron diffraction setup, the elastic diffuse scattering is shown to be a factor of 10⁴ – 10⁷ weaker than the peak intensity of the (002) Bragg reflection. Evidence of inelastic scattering due to phonons is observed, 10⁵ times weaker than the (002) Bragg peak. Our results demonstrate the possibility of using HOPG in complex multiplexing geometries to improve the performance of spectrometers involving multi-analyzer back-end systems, and can be helpful in designing such systems for optimal performance.